As someone who owns a guaranteed week contract at Riviera, it always begins & ends on a Sunday. I assume other resorts with GWs also have it begin & end on Sundays, but I don't know for sure. And I distantly remember them offering some GWs around events like a race weekend, but I don't know the details of those, and I don't think they've offered those in some time.
It does seem quite strange that if you've already booked multiple nights at the 11 month + length of stay point, but can't modify to add one additional night. I don't think that you're blocked by a GW by trying to add a Friday night stay. I'd guess that either something is wonky with the website (maybe try a different web browser or device, or try in an incognito/private browser), or that a room is scheduled to be taken out of inventory for maintenance. I'd recommend checking the website frequently today and tomorrow, and try contacting member services tomorrow if it isn't letting you get the additional night.